Dealing with an arbitrary text file encoded in an unknown way and trying to normalize to UTF-8 (with minimal data loss) is a tricky thing.
This is a little test bed for trying out various detection and transcoding strategies. The problem I was solving had to do with user uploaded CSV or TXT files for importing data into an app. Due to the way excel handles CSVs, I fully expected to deal with UTF-16LE files with a BOM (byte order marker) and/or various flavors of ISO-8859. In a perfect world you would just specify that all files must be valid UTF-8, but most developers don't really understand what UTF-8 is let alone your average user! You can't expect a user to do anything more than hit export on their spreadsheet and dump the resulting mess into your file uploader.
I started my exercise by laying a baseline with 1.9's built in string encoding methods.
Explicitly transcoding the files works flawlessly (in MRI and jRuby) as long as the source encoding was
set to exactly match the file's actual encoding. One trick here is setting the File.open directive
as rb:bom|utf-8. This scraps a BOM (Byte Order Marker) if one is present and sets the
encoding to UTF-8 even if that's not the actual string encoding. Once you have this BOM stripped
string you can do the explicit transcode and everything comes out nice. This is great and all, but
if you don't know the precise source encoding of the file you are dealing with then the results
of the encode might not be so pretty.
In the quest to detect the source encoding I tried several gems. rchardet19 comes very close to getting things right: it nails the unicode files (UTF-8 and UTF-16LE), and at least returns a flavor of ISO-8859 for the windows-1252 and iso-8859-1 files (ISO_8859_8). Unfortunately trying to transcode to UTF-8 and setting the source encoding as 8859-8 yields not so great results. Close but no cigar.
charlock_holmes is the next gem I tried. I expected this one to be the winner as it is built on top of the icu4c which is supposedly the most badass character encoding detector to ever walk these lands. It did not even come close to guessing the encodings correctly. As you can see below it guessed correctly for valid UTF-8 (congrats), binary of all things for UTF-16LE (helpful!), and EUC-JP for the ISO flavored files (WAT?).
Ensure-encoding won the day. One of the strategies this gem employs is very similar to what I was planning on writing by hand: using an educated guess, pick a small subset of encodings and test the unknown string against them one by one until you get a valid encoding. Once the source encoding is establish you can then transcode to UTF-8 successfully. If all your anticipated encodings fail to get a valid match, then you can fall back to an encode without an explicit source set and just pass in the options so that unknown or invalid characters get tossed out or replaced rather than raising an encoding error. Example:
some_string.encode(Encoding::UTF_8, invalid: :replace, undef: replace)As the output below shows, Ensure-encoding yields the same results as my contrived explicit transcode test.
Output:
$>ruby char_test.rb
explicit transcode
------------------------------
test_files/utf8.txt: UTF-8 Unicode text
transcoding from UTF-8 to UTF-8
abcdefghijklmnopqrstuvwxyz
Σὲ γνωρίζω ἀπὸ τὴν κόψη χαῖρε, ὦ χαῖρε, ᾿Ελευθεριά!
Οὐχὶ ταὐτὰ παρίσταταί μοι γιγνώσκειν, ὦ ἄνδρες ᾿Αθηναῖοι,
გთხოვთ ახლავე გაიაროთ რეგისტრაცია Unicode-ის მეათე საერთაშორისო
Зарегистрируйтесь сейчас на Десятую Международную Конференцию по
๏ แผ่นดินฮั่นเสื่อมโทรมแสนสังเวช พระปกเกศกองบู๊กู้ขึ้นใหม่
ᚻᛖ ᚳᚹᚫᚦ ᚦᚫᛏ ᚻᛖ ᛒᚢᛞᛖ ᚩᚾ ᚦᚫᛗ ᛚᚪᚾᛞᛖ ᚾᚩᚱᚦᚹᛖᚪᚱᛞᚢᛗ ᚹᛁᚦ ᚦᚪ ᚹᛖᛥᚫ
valid encoding: true
test_files/utf16le.txt: Little-endian UTF-16 Unicode text
transcoding from UTF-16LE to UTF-8
abcdefghijklmnopqrstuvwxyz
Σὲ γνωρίζω ἀπὸ τὴν κόψη χαῖρε, ὦ χαῖρε, ᾿Ελευθεριά!
Οὐχὶ ταὐτὰ παρίσταταί μοι γιγνώσκειν, ὦ ἄνδρες ᾿Αθηναῖοι,
გთხოვთ ახლავე გაიაროთ რეგისტრაცია Unicode-ის მეათე საერთაშორისო
Зарегистрируйтесь сейчас на Десятую Международную Конференцию по
๏ แผ่นดินฮั่นเสื่อมโทรมแสนสังเวช พระปกเกศกองบู๊กู้ขึ้นใหม่
ᚻᛖ ᚳᚹᚫᚦ ᚦᚫᛏ ᚻᛖ ᛒᚢᛞᛖ ᚩᚾ ᚦᚫᛗ ᛚᚪᚾᛞᛖ ᚾᚩᚱᚦᚹᛖᚪᚱᛞᚢᛗ ᚹᛁᚦ ᚦᚪ ᚹᛖᛥᚫ
valid encoding: true
test_files/iso88591.txt: ISO-8859 text
transcoding from ISO-8859-1 to UTF-8
abcdefghijklmnopqrstuvwxyz
¡¢£¤¥¦§¨©ª«¬&®¯°±²³´µ¶·¸¹º
»¼½¾¿×÷ÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒ
ÓÔÕÖØÙÚÛÜÝÞßàáâãäåæçèéêëìí
îïðñòóôõöøùúûüýþÿ
valid encoding: true
test_files/windows1252.txt: ISO-8859 text
transcoding from Windows-1252 to UTF-8
abcdefghijklmnopqrstuvwxyz
¡¢£¤¥¦§¨©ª«¬&®¯°±²³´µ¶·¸¹º
»¼½¾¿×÷ÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒ
ÓÔÕÖØÙÚÛÜÝÞßàáâãäåæçèéêëìí
îïðñòóôõöøùúûüýþÿ
valid encoding: true
charlock detection
------------------------------
test_files/utf8.txt: UTF-8 Unicode text
charlock:
{:type=>:text, :encoding=>"UTF-8", :confidence=>100}
test_files/utf16le.txt: Little-endian UTF-16 Unicode text
charlock:
{:type=>:binary, :confidence=>100}
test_files/iso88591.txt: ISO-8859 text
charlock:
{:type=>:text, :encoding=>"EUC-JP", :confidence=>50, :language=>"ja"}
test_files/windows1252.txt: ISO-8859 text
charlock:
{:type=>:text, :encoding=>"EUC-JP", :confidence=>50, :language=>"ja"}
rchardet19 detection
------------------------------
test_files/utf8.txt: UTF-8 Unicode text
CharDet:
#<struct #<Class:0x9c7eb33> encoding="utf-8", confidence=0.99>
test_files/utf16le.txt: Little-endian UTF-16 Unicode text
CharDet:
#<struct #<Class:0x263534c1> encoding="windows-1252", confidence=0.5>
test_files/iso88591.txt: ISO-8859 text
CharDet:
#<struct #<Class:0x159576c3> encoding="ISO-8859-8", confidence=0.23461713319342622>
test_files/windows1252.txt: ISO-8859 text
CharDet:
#<struct #<Class:0x3e2dce4e> encoding="ISO-8859-8", confidence=0.23461713319342622>
ensure-encoding transcode
------------------------------
test_files/utf8.txt: UTF-8 Unicode text
valid encoding: true
abcdefghijklmnopqrstuvwxyz
Σὲ γνωρίζω ἀπὸ τὴν κόψη χαῖρε, ὦ χαῖρε, ᾿Ελευθεριά!
Οὐχὶ ταὐτὰ παρίσταταί μοι γιγνώσκειν, ὦ ἄνδρες ᾿Αθηναῖοι,
გთხოვთ ახლავე გაიაროთ რეგისტრაცია Unicode-ის მეათე საერთაშორისო
Зарегистрируйтесь сейчас на Десятую Международную Конференцию по
๏ แผ่นดินฮั่นเสื่อมโทรมแสนสังเวช พระปกเกศกองบู๊กู้ขึ้นใหม่
ᚻᛖ ᚳᚹᚫᚦ ᚦᚫᛏ ᚻᛖ ᛒᚢᛞᛖ ᚩᚾ ᚦᚫᛗ ᛚᚪᚾᛞᛖ ᚾᚩᚱᚦᚹᛖᚪᚱᛞᚢᛗ ᚹᛁᚦ ᚦᚪ ᚹᛖᛥᚫ
test_files/utf16le.txt: Little-endian UTF-16 Unicode text
valid encoding: true
abcdefghijklmnopqrstuvwxyz
Σὲ γνωρίζω ἀπὸ τὴν κόψη χαῖρε, ὦ χαῖρε, ᾿Ελευθεριά!
Οὐχὶ ταὐτὰ παρίσταταί μοι γιγνώσκειν, ὦ ἄνδρες ᾿Αθηναῖοι,
გთხოვთ ახლავე გაიაროთ რეგისტრაცია Unicode-ის მეათე საერთაშორისო
Зарегистрируйтесь сейчас на Десятую Международную Конференцию по
๏ แผ่นดินฮั่นเสื่อมโทรมแสนสังเวช พระปกเกศกองบู๊กู้ขึ้นใหม่
ᚻᛖ ᚳᚹᚫᚦ ᚦᚫᛏ ᚻᛖ ᛒᚢᛞᛖ ᚩᚾ ᚦᚫᛗ ᛚᚪᚾᛞᛖ ᚾᚩᚱᚦᚹᛖᚪᚱᛞᚢᛗ ᚹᛁᚦ ᚦᚪ ᚹᛖᛥᚫ
test_files/iso88591.txt: ISO-8859 text
valid encoding: true
abcdefghijklmnopqrstuvwxyz
¡¢£¤¥¦§¨©ª«¬&®¯°±²³´µ¶·¸¹º
»¼½¾¿×÷ÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒ
ÓÔÕÖØÙÚÛÜÝÞßàáâãäåæçèéêëìí
îïðñòóôõöøùúûüýþÿ
test_files/windows1252.txt: ISO-8859 text
valid encoding: true
abcdefghijklmnopqrstuvwxyz
¡¢£¤¥¦§¨©ª«¬&®¯°±²³´µ¶·¸¹º
»¼½¾¿×÷ÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒ
ÓÔÕÖØÙÚÛÜÝÞßàáâãäåæçèéêëìí
îïðñòóôõöøùúûüýþÿ